Glow Plug Help - PeachParts Mercedes-Benz Forum

subscript · #1 12-05-2006, 01:05 PM

My pre-glow system isn't working. '83 300D OM617
This is the first time I have had any issues with the GP's. Functioned perfectly until a couple days ago.
Cold starts are challenging. I fear for the life of my starter and battery.

Dash light does not come on at all.
Bulb is good.
GP relay is getting 12V at main lead.
Relay's 80A fuse is good.
All GP's are getting equal power (10.9V) with key on.
I can hear the GP relay click off after ~30 seconds.
GP resistance thru harness shows four under 1ohm and one at 2.4ohms.

What else should I check?

vstech · #2 12-05-2006, 01:32 PM

Quote:

Originally Posted by subscript

GP relay is getting 12V at main lead.
Relay's 80A fuse is good.
All GP's are getting equal power (10.9V) with key on.
I can hear the GP relay click off after ~30 seconds.
GP resistance thru harness shows four under 1ohm and one at 2.4ohms.

What else should I check?

it sounds like you have one bad GP. also 10.9 is a little low with just the GP's on. I get 11.4 with all 5 of mine on. you may have a weak battery. one GP bad should not affect the light though. is it starting?
John

Biodiesel300TD · #3 12-05-2006, 01:35 PM

I never have luck checking the GP when they are in the car. I have to pull them out. Did you pull yours out to check or check them in?

subscript · #4 12-05-2006, 01:48 PM

That 10.9V GP reading was taken just after an attempted cold start. Battery is new and strong (enough).

It starts, with much cranking and no assistance from the plugs. (Starts easily when warm.)

Everything worked perfectly until a couple days ago when the dash light quit coming on. Now the plugs are not working.

subscript · #5 12-05-2006, 01:52 PM

Quote:

Originally Posted by Biodiesel300TD

I never have luck checking the GP when they are in the car. I have to pull them out. Did you pull yours out to check or check them in?

I have not pulled them yet.

I assumed the problem lies somewhere in the control system since they all quit working at the same time.

vstech · #6 12-05-2006, 02:16 PM

Quote:

Originally Posted by subscript

I have not pulled them yet.

I assumed the problem lies somewhere in the control system since they all quit working at the same time.

you keep saying they quit working... from the meter readings you report, only one is not working.
John

subscript · #7 12-05-2006, 02:20 PM

Quote:

Originally Posted by vstech

you keep saying they quit working... from the meter readings you report, only one is not working.
John

They produce no results. They are not heating my air/fuel mixture.
I went from 3-click cold starts to minute-plus cranking.

vstech · #8 12-05-2006, 02:23 PM

Ahh, you mean the car won't start, not the GP's aren't heating. ok. I would order a full set of new bosch GP's and pop em in. while they are out, you may want to run a compression check on her. also pull the Valve cover and check the cam clearances. then report back with the findings.
John

Biodiesel300TD · #9 12-05-2006, 02:24 PM

Quote:

Originally Posted by subscript

That 10.9V GP reading was taken just after an attempted cold start. Battery is new and strong (enough).
It starts, with much cranking and no assistance from the plugs. (Starts easily when warm.)
Everything worked perfectly until a couple days ago when the dash light quit coming on. Now the plugs are not working.

Are you sure that all the plugs aren't working. It can take only one bad one to cause starting problems depending on the condition of the engine. Have you checked each GP seperately to ensure each one is getting juice? If yes, then I would venture that you have a bad plug or two. If no, then check that and go from there.

networkboy · #10 12-05-2006, 02:27 PM

Quote:

Originally Posted by subscript

They produce no results. They are not heating my air/fuel mixture.
I went from 3-click cold starts to minute-plus cranking.

Try the following:

Wait normal glow time then cycle the key all the way off and back to glow, do you get a light then?

Also:
Brutal start this morning. only 33F :(
This thread is mine and I had almost spot on the same symptoms you are describing. One bad plug was the issue (#4 for me)
A good way to test the plugs in-block is as follows:
Unplug the harness from the relay,
Measure block to threaded tip on each GP, should read ~1 ohm each.
Then measure block to each conductor in the relay plug you removed, should get nominally the same reading.
-nB

subscript · #11 12-05-2006, 02:35 PM

GP resistance was checked at plug and at relay harness. 1 plug was not the same (2.4 ohms).

I have not pulled any GPs to see if they get hot.
I can tell by the way it tries to start that no more than one plug is effective.

Dash light does not come on, no matter what sequence is followed (bulb is good though).

I am a starving student - no money to throw a set of new plugs at it now.

kerry · #12 12-05-2006, 03:04 PM

Check to see if you are getting power at the glowplugs and at the relay when the plugs are activated. You could also try jumping 12volts directly to the 80 amp fuse and see if that causes them to work. If so, the problem is upstream from the fuse. But as a starving student, you could always use the jumper wire to heat them up and get is started until further investigation and parts purchase could be done.

subscript · #13 12-05-2006, 03:07 PM

Quote:

Originally Posted by kerry edwards

Check to see if you are getting power at the glowplugs and at the relay when the plugs are activated. You could also try jumping 12volts directly to the 80 amp fuse and see if that causes them to work. If so, the problem is upstream from the fuse. But as a starving student, you could always use the jumper wire to heat them up and get is started until further investigation and parts purchase could be done.

Plugs and relay getting power.
I'll try the hotwire method tomorrow for shirts and giggles.

fireman1073 · #14 12-05-2006, 03:10 PM

My 83 300d did the same thing.

No glow plug light but i could hear the relay click off after like 30 seconds.

I had two bad glow plugs.

replaced only the bad ones.

$9.99 at auto zone

Steve

kerry · #15 12-05-2006, 03:11 PM

If the plugs are getting power, then the earlier poster who said one bad plug can stop if from starting should be the ruling authority on this question. Buy the plug and replace the bad one, no other choice.